Distillation of benzene fractions



March 9, 1943. R B GREENBURG 2,313,538

DISTILLATION vOF BENZENE FRACTIONS Filed July 18, 1941 ATTO R N EYPatented Mar. 9, 1943 DISTILLATION OF BENZENE FRACTIONS Richard B.Greenburz, Philadelphia, Pa., assignor, by mesne assignments, to AlliedChemical &

Dye Corporation Application July 18, 1941, Serial No. 402,951

` (C1. 2oz-42) 4 Claims.

This invention relates to a process for recovering benzene from mixturescontaining it and other hydrocarbons.

Numerous hydrocarbon oils are known which contain benzene in varyingproportions. Gasoline fractions obtained by the distillation of certaintypes of petroleum or recovered from the products of catalyticreformation processes, frequently contain substantial proportions ofbenzene, although mainly consisting of other hydrocarbons. Oils ofpetroleum origin having a. considerable content of aromatics, includingbenzene, may be treated by well known selective solvent processes toproduce fractions rich in aromatics; for example, extraction of suitablefractions of such petroleum oils with sulfur dioxide may yield fractionsof high benzene content. In such cases benzene is accompanied bynon-aromatic hydrocarbons which may be largely olefinic, parafnic, ornaphthenic in character. A considerable proportion of these hydrocarbonscannot be completely separated from the benzene by direct fractionaldistillation because of the closeness of their boiling points to that ofbenzene or because they form constant boiling mixtures with benzene.Furthermore, while ordinarily benzene is readily separable by directfractional distillation from light oils produced by the gasification ofcoal, in some cases the benzene is accompanied by diicultly separablenon-aromatic hydrocarbons of the same general character as described,owing to carbonization conditions, type of coal used or other specialcircumstances. Also synthetic hydrocarbon oil mixtures produced byvarious catalytic processes may contain benzene which, when recovered,is accompanied by similar difcultly separable constituents.

By fractional distillation of these oils containing benzene, fractionsrelatively high in benzene content may be obtained. These benzenefractions, however, will still contain large amounts of the otherconstituents of the oil having boiling points in the neighborhood of theboiling point of benzene or forming mixtures of constant boiling pointsin the range of temperatures at which benzene distills from the oil. (Inthis specification and claims, I refer to such diicultly,

presents a diflicult problem of great industrial importance.

For many purposes to which benzene is put, it is desirable to use aspure a material as may be economically available. For example, benzeneis used for the production of pharmaceutical chemicals, dyes andsynthetic phenol. For these purposes a pure benzene is preferred. Directfractional distillation, because of the diiculties pointed out above,will not effect a recovery of pure benzene separate from like-boiling,nonaromatic hydrocarbons such as enumerated, and in many cases will notgive fractions of suitably high benzene content or free from materialswhich even in small concentrations adversely affect the processes bywhich the benzene is treated or the products produced therefrom.

It is an object of my invention to provide a process whereby benzene ofany desired degree of purity may be recovered from hydrocarbon oilmixtures comprising non-aromatic hydrocarbons from which the benzene isnot separable by a conventional distillation.

In using my invention for the treatment of an oil in which the benzeneconcentration is low or when the oil is one containing materials of wideboiling range, I prefer first to fractionally distill the oil torecover` therefrom an enriched benzene fraction. Although benzenefractions having an end boiling point substantially above the boilingpoint of benzene (e. g., a boiling point up to about 85 C.) may beazeotropically distilled in the manner hereinafter described, I preferthe benzene fraction treated by my process to be one having an endboiling point substantially corresponding to the boiling point of purebenzene, e. g., an end boiling point of about 80 C. Further, for thereasons which will be more specifically pointed out below, I prefer thata benzene fraction recovered by a preliminary distillation of a crudebenzene oil be so cut as to exclude therefrom the forerunnings which donot contain substantial proportions of benzene; for example, to excludesubstantial proportions of oils distilling below C.

In this specification and claims, the term benzene fraction is usedbroadly to designate any oil fraction containing both benzene and otherhydrocarbons which, under the conditions maintained in directdistillation processes as commonly used. for distilling oils, distillfrom the fraction within the same temperature range as does the benzene.

I have discovered that benzene may be separated from a benzene fractionsuch as described above by subjecting it to azeotropic distillation inthe presence of ethyl formate. 'I'he non-aromatic hydrocarbons, which inthe absence of the ethyl formate distill from the benzene fraction inthe same temperature range as the benzene and hence are not separabletherefrom by direct fractional distillation, may be distilled from thebenzene fraction in the presence of the ethyl formate to leave a residuecontaining benzene of any desired purity with respect to its content ofthese like-boiling, non-aromatic hydrocarbons. On fractionallydistilling a mixture of the ethyl formate with a benzene fraction, thematerial first to distill over is a mixture of low-boiling azeotropesconsisting chieiiy of ethyl formate and non-aromatic hydrocarbons(together with water if present). On continued distillation, thenon-aromatic hydrocarbons are selectively removed and a residue enrichedin benzene is obtained. The distillate will contain ethyl formate andthe non-aromatic hydrocarbons distilled from the benzene fraction. Itmay also contain some benzene carried over in the distillation.

The azeotropic distillation may be carried out either under anhydrousconditions or in the presence of substantial quantities of Water. Whenwater is present a part or all of the water, depending upon the quantitypresent, is carried over in the distillate as an azeotrope of the water,ethyl formate and non-aromatic hydrocarbons. Water appears to aid in theproduction ci a pure benzene product. By washing the distillate with anaqueous solution oi sodium acid sulflte the ethyl formate in thedistillate may be separated in an aqueous phase from an oil phasecontaining the hydrocarbons carried over into the distillate. Theaqueous phase may be treated to recover ethyl formate therefrom forreturn to the distillation and reuse in the azeotropic distillation ofadditional benzene fraction. Ethyl formate which may be left in theresidue may be removed by washing the residue with sodium acid sultesolution. The oil phase thus obtained, containing benzene which has beenseparated from non-aromatic hydrocarbons in the original benzenefraction, may then be washed with caustic soda solution to remove sulfurdioxide and, if desired, may be given a conventional acid wash andredistillation to further purify the benzene.

My invention comprises, therefore, azeotropically distilling in thepresence of ethyl formate a benzene fraction containing non-aromatichydrocarbons which distill from said fraction in the absence of anazeotropic agent, in the same temperature range as the benzene, e. g.,hydrocarbons which normally boil from mixtures thereof with benzenebetween 70 and 80 C. and from which the benzene is not readily separableby conventional distillation.

The azeotropic distillation in the presence of the ethyl formate shouldbe carried out with rectification of the vapors. Sufcient ethyl formateshould be present to carry over as azeotropes the non-aromatichydrocarbons so that the residue containing benzene is freed to thedesired degree of these non-aromatic hydrocarbons. The quantity of ethylformate thus required will, of course, depend upon the amount of benzenefraction distilled, the quantity of non-aromatic hydrocarbons containedtherein, etc. Any excess of ethyl formate present will not materiallyinterfere with the separation of the non-aromatic hydrocarbons from thebenzene, although it is obvious that a large excess is undesirable inthat it requires removal and recovery of the ethyl formate from thebenzene product of the process.

As pointed out above, it is highly desirable to recover from benzenefractions a relatively pure benzene product and the process of myinvention permits of obtaining an enriched benzene residue of anydesired purity with respect to hydrocarbons which, in the absence of anazeotropic agent, distill from the benzene fraction in the sametemperature range as the benzene. My invention contemplates, therefore,continuing the azeotropic distillation of the benzene fraction under theconditions described above until the distillation residue is enriched toa desired extent in benzene with respect to other likeboilinghydrocarbons; i. e., hydrocarbons which will distill from the benzenefraction, in the absence o! an azeotropic agent, in the same temperaturerange as the benzene distills therefrom. In practicing my invention thedistillation preferably is controlled to obtain a benzene residue whichcontains at least 90 parts by weight of benzene to l0 parts oflike-boiling, non-benzene hydrocarbons.

'I'he benzene fraction treated in accordance with my inventionpreferably will contain little, if any, hydrocarbons distilling from thehydrocarbon-benzene fraction at temperatures ma.- terially above thoseat which benzene distills therefrom. On the other hand, high boilingmaterials may be left with the benzene residue at the conclusion of theazeotropic distillation of the benzene fraction under the conditions setforth above to separate the benzene from the hydrocarbons of similarboiling range. Once this separation has been effected, the benzene maybe separated from the high boiling hydrocarbons by fractionaldistillation in the absence of an azeotropic agent. Whether or not high.boiling constituents should be left in the benzene fraction to beazeotropically `distilled or whether, if left in the benzene fraction,they will remain in the residue after azeotropic distillation, dependsupon a number of factors, among which their boiling range and chemicalcharacteristics are important. If the high boiling constituents formazeotropes with the agent used which have boiling points close to thetemperatures at which benzene distills over, it is preferable toseparate such high boiling constituents from the benzene fraction by adirect fractional distillation before azeotropically distilling thebenzene fraction.

When pure benzene is to be recovered it is preferred the benzenefraction subjected to azeotropic distillation in accordance with myinvention be a fraction boiling in the range of 65 C. to C. Such afraction may be azeotropically distilled by my process and pure benzeneobtained with a relatively small quantity of azeotropic agent presentduring the distillation. Pure benzene may be obtained by distilling withan azeotropic agent a wider boiling benzene fraction but the quantity ofazeotropic agent present in the distillation of the benzene fraction ofwider boiling range must be substantially increased as compared with thequantity which sumces for distilling the fraction of the narrowerboiling range.

Benzene of high purity is desired for nitration and other purposes andthe process of my invention provides a method whereby a pure benzene ofnitration grade may be economically recovered from benzene oils. Theinvention, however, is

not limited thereto. Benzene products of lower purity than nitrationgrade benzene are industrially used for various purposes, e. g., assolvents. The process of my invention may be used advantageously forrecovering such products from hydrocarbon oils of lower benzene content.My process is particularly advantageous when the .benzene is to beseparated from most of the likeboiling, non-aromatic hydrocarbonspresent in an oil fraction together with benzene, e. g., when a. productis to be obtained containing benzene and no more than 10 parts by Weightof like-boiling, non-aromatic hydrocarbons for every 90 parts by weightof benzene.

My invention will be more particularly described and illustrated by thefollowing example of a process for the azeotropic distillation of abenzene fraction in the presence of ethyl formate.

The apparatus used for carrying out the process of this example,diagrammatically illustrated in the accompanyingI drawing, comprised astill I with heater 2 and rectification column 3 of conventional design.A condenser 4 was provided to which the vapors from the top of thecolumn were led and in which they were cooled and condensed. A pipe 5was provided for returning condensate from the condenser to the top ofthe rectification column to serve as reflux for the column. A secondpipe 6 was also provided for drawing oi from the condenser a portion ofthe condensate formed therein. Pipes l and 8 are provided for theintroduction of liquids into still I. A pipe 9 serves for Withdrawal ofresidue from this still.

The above-described apparatus was employed for the distillation of abenzene fraction having a boiling range of about 75 C. to about 80 C.and containing 69% by volume of benzene, the remainder principallyconsisting of like-boiling parains.

100 volumes of this benzene fraction, 100 volumes of ethyl formate, and10 volumes of water were charged into the still of the apparatusdescribed above. This charge was boiled in the still and the vaporspassing therefrom to the fractionating column were rectified in thiscolumn. The vapors leaving the top of the column were condensed, a partof the condensate was withdrawn as distillate through pipe 6, and theremainder of the condensate returned as reflux through pipe 5 to the topof the fractionation column. Distillation started with the temperatureof the vapors at the top of the fractionation column at about 46 C. Asthe distillation proceeded, the temperature rose quickly to 50 C. andthen gradually rose to about 52.5 C. At this stage of the process thenon-aromatic hydrocarbons were largely distilled over as azeotropes withthe ethyl formate. As the distillation proceeded, the non-aromatichydrocarbons became exhausted and the temperature then rapidly rose to63.0 C., at which point the distillation was discontinued. A total of125 volumes of distillate was taken off through pipe 6. A residue of 68volumes was left in still l. 'I'his residue was washed with a solutionof sodium acid sulte, followed by washing with caustic soda solution andthe oil separated from the aqueous solutions, amounting to 54 volumes,was analyzed for benzene. This oil was found to contain 98% benzene bythe specific dispersion method for its analysis described in Industrial8: Engineering Chemistry, analytical edition, vol.

11, Page 614, November 15, 1939. By the Kattwinkel method of analysis,Brennstoff-Chem.. vol. 8, page 353 (1927), it was shown as benzene.

The process of this example may be modied to carry out the distillationof the benzene fraction and ethyl formate under substantially anhydrousconditions; l. e., without adding to the still charge in the aboveexample the 10 volumes of water. Under these conditions distillationstarted at about 47 C. The temperature rose quickly to about 52.8 C. andthen gradually rose to about.

53.9 C. At this stage the non-aromatic hydrocarbons were largelydistilled over as azeotropes with the ethyl formate. The temperaturethen quickly rose to 79.8 C., at which point the distillation wasdiscontinued. A total of volumes of distillate was thus taken offthrough pipe B. The residue in the still amounted to 68 volumes. Afterwashing this residue with sodium acid sulflte and caustic sodasolutions, analysis of the oil thus recovered showed it contained atleast 93% benzene.

While the example described above employs a batch operation for thedistillation of the benzene fraction, this distillation may be carriedout continuously. For example, ethyl formate and benzene fraction may becontinuously supplied to a column still in which the benzene isconcentrated and, flowing to the bottom of the still, is continuouslyWithdrawn therefrom. The azeotropes of ethyl formate and non-aromatichydrocarbons are continuously vaporized and withdrawn as distillate fromthe top of the distillation column. Suflicient ethyl formate should besupplied to carry over as distillate all of the hydrocarbons to bedistilled from the benzene fraction and separated from the benzene. Byobservation of the vapor temperatures at a suitable point in the vaporrectification column, one may ascertain whether adequate ethyl formateis present. Whenever these vapor temperatures tend to exceed about 54C., either under anhydrous conditions or when water is present, beforethe benzene has been separated to the desired degree from thelike-boiling, non-aromatic hydrocarbons, by supplying additional ethylformate (with or without Water) to the still or rectification column thedesired separation of the benzene and like-boiling, non-aromatichydrocarbons may be accomplished. This point of temperature observationin the apparatus in which the above example of the process was carriedout, is preferably the top of the recticatlon column. One skilled in theart of fractional distillation will recognize for any particular type ofapparatus suitable points for this temperature control.

While in the example given the benzene has been recovered asdistillation residue, the distillation may be continued after theremoval of non-benzene constituents so that an enriched benzene productis recovered as overhead. The distillate may be treated to recover thebenzene product separate from any ethyl formate taken over in distillingoi the enriched benzene product. Furthermore, the distillation may becarried out as a continuous procedure instead of the batch proceduredescribed above. In a continuously operating process enriched benzeneproducts may be taken olf from the bottom of the still or as one or moreside streams from the fractionation column below lthe point of feedingthe benzene fraction thereto. These modifications are within the scopeof my invention.

.In this specification I have described the azeotrop'ic distillation asbeing carried out under substantially atmospheric pressure. Thetemperatures as given herein are corrected temperatures for 1 atmosphereabsolute (760 mm. of Hg). When carried out under pressures other thanatmospheric, the temperature conditions will differ from those given. Inany given case, however, the temperatures will correspond to the changein boiling points of the materials due to the difference in pressure.

I claim:

1. The process for the recovery of benzene from a benzene fractioncontaining the same and containing a mixture of like-boiling,non-aromatic hydrocarbons which comprises distilling said benzenefraction and rectifying the distilled vapors in the presence of ethylformate in an amount which selectively carries over as distillate saidlike-boiling, non-aromatic hydrocarbons and leaves a hydrocarbon residueof the distillation enriched in benzene.

2. The process for the recovery of benzene which comprises distilling abenzene oil fraction boiling within the range 65 C. to 80 C. andcontaining benzene and a mixture of like-boiling, non-aromatichydrocarbons and rectifying the distilled vapors in the presence of aquantity of ethyl formate which selectively carries over as distillatesaid like-boiling, non-aromatic hydrocarbons and leaves a hydrocarbonresidue of the distillation enriched in benzene.

3. The process for the recovery of benzene from a benzene fractioncontaining the same and containing a mixture of like-boiling,non-aromatic hydrocarbons which comprises distilling said benzenefraction and rectifying the distilled vapors in the presence of ethylformate and water, said ethyl formate being present in an amount whichselectively carries over as distillate said like-boiling, non-aromatichydrocarbons and leaves a hydrocarbon residue of the distillationenriched in benzene.

4. The process for the recovery of benzene from a benzene fractioncontaining the same and containing a mixture of like-boiling,non-aromatic hydrocarbons which comprises distilling said benzenefraction and rectifying the distilled vapors in the presence of ethylformate, said ethyl formate being present in amount such that at a pointin the fractionation of the vapors evolved in distilling the benzenefraction the temperatures does not exceed about 54 C. until saidlike-boiling, non-aromatic hydrocarbons have been vaporized leaving ahydrocarbon residue of the distillation containing benzene and no morethan 10 parts by weight of like-boiling, non-aromatic hydrocarbons forevery 90 parts by weight of benzene.

RICHARD B. GREENBURG.

